The present invention is directed towards filter paper and a method of making the same. The filter paper is comprised of a flat annular disc of paper which is impregnated with a phenolic resin. Additionally, a method is set forth explaining the process of making the impregnated filter paper.
The filter paper when stacked is suitable for use in filter devices such as set out in U.S. Pat. No. 2,773,602. This particular type of filter is called a "mechanical or edge filter". An edge filter utilizes a plurality of concentric flat annular discs stacked one upon another and supported by a tube which engages the inner annular ring of the disc. Between the discs is a small space or interstice. The internal tube is porous such that a fluid may permeate through the wall into the inner cavity of the tube. The edge filter operates by introducing the liquid material to be filtered to the area outside of the stack of annular discs. The liquid is then forced between the interstices of the stacked discs where particles are mechanically blocked in the interstices. Additionally, particles which would normally pass through the interstices may be captured on the plane surfaces of the discs. The filtered liquid may then penetrate the internal tube and be removed from the filter device through the cavity in the tube.
The performance of a filter is determined by the filter's ability to remove particles from the liquid and the flow rate of material through the filter.
Particle removal ability can be further viewed as (a) the filter's ability to remove all suspended solids from the liquid, and (b) the filter's ability to remove suspended solids of a particular particle size. Of special concern in the area of edge filter, is the filter's ability to remove suspended solids of a size which would normally pass through the interstice. To accomplish the capture of those solid particles which would pass through the interstice, a disc with a coarse surface may be used.
The flow rate through the filter device depends upon the pressure drop across the filter. Across the filter meaning from outside of the annular discs to the inside cavity of the tube. Increased pressure drop due to fouling of the interstices and swelling of the paper, causes reduced flow through the filter. Three primary causes of increased pressure are (a) fouling of the interstices with filtered particles, (b) swelling of the filter paper due to absorption of the filtered liquid into the paper, and (c) reduction of the size of the interstices due to compression of the annular discs by the compressive forces from the surrounding liquid. Thus, to maintain the flow through the filter it is imperative to minimize the increase in pressure drop across the filter caused by swelling and compression of the interstices.
Filter discs used in prior art edge filters, as described above, are made of metal, plastic, paper or fibrous material. Metal or plastic edge filters are able to withstand the compressive force exerted by the surrounding liquid thus reducing increased pressure drop due to compression of the discs. But the surfaces of the metal discs are smooth, thus unable to capture particles too small to be blocked by the interstices. Thus, effectiveness of removability is reduced because no coarse surface is provided to catch small particles.
Edge filters made of paper or fibrous material are able to entrap particles too small to be blocked in the interstices. This is due to the coarse surfaces on the plane portions of the discs. But paper or fibrous material is unable to withstand the compressive force of the liquid. The compression due to the surrounding liquid will cause the interstices to close.
Paper and fibrous materials have the ability to absorb the liquid. Absorption of the liquid causes fibers to swell. The swelling adversely affects filter operation by causing the interstices to close and reducing the ability to entrap small particles on the plane surfaces of discs.